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Oxy-fuel combustion study of biomass fuels in a 20 kWth fluidized bed combustor
Sher, F., Pans, M. A., Sun, C., Snape, C. & Liu, H. Author post-print (accepted) deposited by Coventry University’s Repository Original citation & hyperlink:
Sher, F, Pans, MA, Sun, C, Snape, C & Liu, H 2018, 'Oxy-fuel combustion study of biomass fuels in a 20 kWth fluidized bed combustor' Fuel, vol. 215, pp. 778-786. https://dx.doi.org/10.1016/j.fuel.2017.11.039
DOI 10.1016/j.fuel.2017.11.039 ISSN 0016-2361 ESSN 1873-7153 Publisher: Elsevier NOTICE: this is the author’s version of a work that was accepted for publication in Fuel. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Fuel, 215 (2018) DOI: 10.1016/j.fuel.2017.11.039 © 2017, Elsevier. Licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ Copyright © and Moral Rights are retained by the author(s) and/ or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This item cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder(s). The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. This document is the author’s post-print version, incorporating any revisions agreed during the peer-review process. Some differences between the published version and this version may remain and you are advised to consult the published version if you wish to cite from it.
1
Oxy-fuel combustion study of biomass fuels in a 20 kWth fluidized 1
bed combustor 2 3
Farooq Sher, Miguel A. Pans, Chenggong Sun, Colin Snape, Hao Liu* 4
Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK. 5
Abstract 6
Oxy-fuel combustion is one of the promising carbon capture technologies considered to be suitable for 7
future commercial applications with stationary combustion plants. Although more and more biomass 8
and waste are now being burned in stationary combustion plants, research on oxy-fuel combustion of 9
biomass has received much less attention in comparison to oxy-fuel combustion of coal. In this work, a 10
series of tests was carried out in a 20 kWth fluidized bed combustor under oxy-fuel conditions firing 11
two non-woody fuels (miscanthus and straw pellets) and one woody fuel (domestic wood pellet). The 12
effects of the combustion atmosphere (air and oxy-fuel) and oxygen concentration in the oxidant of the 13
oxy-fuel combustion on gas emissions and temperature profiles were systematically studied with the 14
overall excess oxygen coefficient/oxygen level in the reactor being maintained roughly constant 15
throughout the tests. The experimental results showed that simply replacing the air with an oxy-fuel 16
oxidant of 21% O2 and 79% CO2 resulted in a significant decrease in combustion temperature and 17
ultimately led to the extinction of the biomass flame due to the larger specific heat of CO2 compared to 18
N2. To keep a similar temperature profile to that achieved under the air combustion conditions, the 19
oxygen concentration in the oxidant of O2/CO2 mixture had to be increased to 30 vol.%. A drastic 20
decrease in CO emissions was observed for all the biomass fuels (up to 80% reduction when firing 21
straw) under oxy-fuel combustion conditions providing that the oxygen concentration in the oxidant of 22
O2/CO2 mixture was above 25 vol.%. NOx emissions were found to decrease with the oxygen 23
concentration in the oxy-fuel oxidant, due to i) the increase of bed temperature, which implies more 24
volatile-N released and converted in the highly-reducing dense bed zone and ii) the less dilution of the 25
gases inside the dense bed zone, which leads to a higher CO concentration in this region enhancing the 26
*Corresponding author. Tel.: +44 (0) 115 84 67674; Fax: +44 (0) 115 95 13159
E-mail address: [email protected] (F. Sher), [email protected] (H.Liu)
2
reduction of NOx. Similar NOx emissions to those obtained with air combustion were found when the 27
oxygen concentration in the oxy-fuel oxidant was kept at 30%. Further analysis of the experimental 28
results showed that the gas emissions when firing the non-woody fuels were controlled mainly by the 29
freeboard temperature instead of the dense bed region temperature due to the characteristic high 30
contents of volatile matter and fines of this kind of biomass fuels. 31
32
Key words: Biomass combustion; Fluidized bed combustion; Oxy-fuel combustion; Carbon capture; 33
NOx and CO emissions 34
35
1. Introduction 36
Growing concerns on the greenhouse gas emissions and their potential impact on climate change 37
demand not only the application of CO2 capture and storage (CCS) technologies to large point 38
anthropogenic CO2 emission sources such as coal and natural gas fired power plants but also the 39
implementation of CO2 negative combustion technologies such as Bio-energy with Carbon Capture and 40
Storage (BECCS) within the next decades. Although removing CO2 from the atmosphere, i.e. direct 41
capture of CO2 from air, may be necessary in the longer term, direct CO2 capture is more technically 42
challenging and more expensive than CCS and BECCS applied to large scale combustion plants[1-3]. 43
Biomass is considered as a renewable fuel, a carbon-neutral energy source and hence its combustion 44
integrated with CCS can lead to negative CO2 emissions. Biomass has already captured worldwide 45
attention in the context of greenhouse gas control even though fossil fuels are expected to retain their 46
dominant role in the world energy supply in the coming decades[4]. 47
48
Oxy-fuel combustion is one of the most developed CCS technologies and considered as technically 49
feasible and economically competitive for future commercial applications [5-7]. Oxy-fuel combustion 50
refers to fuel being burned in a mixture of oxygen and recycled flue gas (RFG). Unlike conventional 51
air combustion plants that use air as the oxidant, an oxy-fired plant employs an Air Separation Unit 52
(ASU) to produce an almost pure oxygen stream. The oxygen stream is then combined with RFG to 53
3
produce an oxygen enriched gas as the oxidant. The flue gas recycle is necessary to moderate the 54
otherwise excessively high flame temperature that would result from fuel combustion in pure oxygen. 55
After the removal of water and other impurities from the flue gas exhaust stream, high-purity CO2 (up 56
to 95%) is produced and almost ready for sequestration[8, 9]. As mentioned above, the combination of 57
oxy-fuel combustion with biomass could effectively provide a method which would not only avoid 58
further CO2 emissions but also helps reduce the atmospheric CO2. Furthermore, the oxy-fuel process 59
also offers other advantages such as improving the ignition and burnout performance. 60
61
Among all the available combustion technologies, fluidized bed combustion (FBC) is often considered 62
as the best choice for the combustion and/or co-combustion of biomass, waste and other low quality 63
solid fuels due to its fuel flexibility, long residence times, and uniform combustion temperatures. The 64
characteristics of FBC also offers several advantages for its application in oxy-fuel systems [10]. Firstly, 65
the difficulty of flue gas recirculation for temperature control in pulverized fuel (PF) applications could 66
be reduced in circulating fluidized bed (CFB) by means of the bed material recirculation, since the 67
specific heat of the solids is much higher than that of the recycled flue gas. Secondly, lower NOx 68
emissions and better sulphur removal are possible. Finally, it is easier to retrofit an FB boiler from air 69
to oxy-fuel combustion as there will be no need for a new burner. 70
71
The effects of oxy-fuel atmosphere and O2 concentration in the oxy-fuel oxidant gas on pollutant 72
emissions (NOx and CO) in fluidized bed systems firing different kinds of coal have been thoroughly 73
investigated during the past years by a number of researchers. In general, the experimental results 74
showed that NOx emissions in oxy-fuel combustion with low O2 concentrations are lower than those 75
obtained under air-firing atmosphere, because of the lower temperatures as well as higher char and CO 76
concentrations in the dense bed [10, 11]. Furthermore, NOx emissions were found to increase with the 77
increasing O2 concentrations in the oxy-fuel oxidant, which is as a result of i) the increase of the 78
temperature in the furnace which elevates the concentrations of O and OH radicals and enhances NO 79
formation and ii) the lower gas velocity in the riser and longer residence time of fuel particles in the 80
combustor, which may promote the fuel-N conversion into NOx precursors [10-14]. In some works an 81
4
opposite trend was found, i.e. an decrease of NOx with the increasing O2 concentration in the oxy-fuel 82
oxidant, as in the study of de las Obras-Loscertales et al. [15]. The authors explained this trend by means 83
of the different operational procedure used, comparing with other works: in their work [15], an increase 84
in the oxygen concentration was compensated with an increase of the coal flow rate fed to the reactor, 85
keeping constant the total gas flow rate and excess oxygen coefficient in all tests. As a result, more 86
unconverted char was present in the bed, favouring the NO reduction on the char surface. 87
88
Regarding CO emissions, Duan et al. [16] observed a much lower CO emission in air than those in an 89
oxy-fuel atmosphere with the same O2 concentration when firing two kind of coal in a 50 kWth CFB 90
facility, due to the higher temperature achieved under the air combustion conditions. They also reported 91
a decrease in CO emissions when the O2 concentration in the oxy-fuel oxidant gas increased, as the 92
oxidation of carbon was more complete and therefore less CO was formed. On the other hand, Hofbauer 93
et al. [14] observed similar CO emissions for air combustion and two oxy-fuel cases investigated (with 94
26 vol.% and 36 vol. % of O2 in the oxidant, respectively), firing bituminous coal in a 150 kWth CFB 95
reactor. Jia et al. [17] performed a series of oxy-fuel tests with flue gas recycle in a 100 kWth CFB 96
combustor firing bituminous coal. The CO emissions of oxy-fuel combustion were found to be equal or 97
slightly lower than those of air firing, due mainly to the higher cyclone temperature achieved with oxy-98
fuel combustion. 99
100
Biomass properties differ from those of coal in many important ways which results in different 101
combustion behaviours [18]. For example, biomass generally has less carbon, more oxygen, higher 102
hydrogen content and lower heating value. There are huge differences in volatile matter contents 103
between biomass and coal: biomass can lose up to 90% of their masses (as volatiles) in its first stage of 104
combustion, much higher than any ranks of coal (from less than 10% for anthracite to ca. 40% for high-105
volatile bituminous coals) [19, 20]. The effects of oxy-fuel combustion conditions on the combustion 106
performance and emissions of biomass fuels are expected to be differing from those of coal as a result 107
of the differences in properties between biomass and coal. So far, few have investigated oxy-fuel 108
combustion in fluidized bed reactors firing 100% biomass fuels and therefore further research is still 109
5
needed. Duan et al. [21] conducted a series of experiments firing three kinds of Chinese biomass, i.e. 110
rice husk, wood chips and dry wood flour, under air and oxy-fuel atmosphere in a 10 kWth CFB 111
combustor. The main objective of this study was to investigate the pollutant emissions of the co-firing 112
of biomass with coal under oxy-fuel combustion conditions although experiments firing only the 113
biomass fuels were also carried out for comparison purposes. They observed lower NO emissions in 114
the oxy-fuel atmosphere compared with those with air combustion. This behaviour was explained as 115
the result of the reduced yield of NOx precursors like NH3 during the devolatilization process and the 116
enhanced NO reductions via char/NO/CO reactions under the oxy-fuel combustion conditions. They 117
also concluded that the NO emission increased with the bed temperature, overall oxygen concentration 118
and the primary oxidant fraction when co-firing biomass and coal with a mixing ratio of 0.2 in oxy-fuel 119
combustion. However, the effects of the oxygen concentration in the oxy-fuel oxidant gas on the gas 120
emissions and temperature profiles firing 100% biomass fuels were not investigated in this study. 121
122
The objective of the present work is to continue delving into the barely-studied oxy-fuel combustion of 123
biomass fuels firing three kind of biomass fuels, two non-woody (miscanthus and straw) and one woody 124
(wood), in a 20 kWth bubbling fluidized bed (BFB) combustor, studying the effects of the combustion 125
atmosphere (air and oxy-fuel) and the oxygen concentration in the oxy-fuel oxidant on the gas emissions 126
and temperature profiles. 127
2 Experimental 128
2.1 Experimental setup 129
The experimental system, shown in Fig. 1, mainly includes a BFB combustor (20 kWth) and the auxiliary 130
systems for air supply, biomass feeding, and gas analysis. The combustor consists of a stainless steel 131
reactor of 102 mm i.d. and 800 mm height, a freeboard of 154 mm i.d. and 1100 mm height, and a 132
plenum of 102 mm i.d. and 300 mm height. A water cooled heat extraction probe located inside the bed 133
allows the bed temperature to be controlled by means of the extraction of heat from the reactor. This 134
probe can be moved vertically along the reactor to change the contact surface inside the reactor to 135
prevent the bed from reaching very high temperature values and thus avoid agglomeration and 136
6
defluidisation of the bed particles. Furthermore, the cooling water flow rate of the probe can be adjusted 137
to control the heat extraction and hence the combustion temperature inside the reactor. 138
139
As the real flue gas recirculation is not included with the experimental system, a premixed flow of CO2 140
and O2 which are supplied from gas cylinders and monitored by rota meters (calibrated for O2 and CO2 141
respectively) is used as the main oxidant for the oxy-fuel combustion and is also used as the fluidising 142
gas of the BFB reactor. For the conventional air combustion tests, compressed air is used as the main 143
oxidant and fluidising gas. The air flow rate is monitored by a rotameter that is calibrated for air. For 144
both oxy and air combustion tests, a small flow of compressed air is also fed through the biomass feeder 145
hopper to prevent backfire and to stop the sand particles coming into the feeding pipe. The oxidant gas 146
is fed into the reactor through a porous stainless gas distribution plate with 100 µm pore size and 12 147
mm thickness. An electric air pre-heater before the plenum and two electric half-cylindrical ceramic 148
radiant heaters surrounding the main bed area are used to preheat the combustion/fluidisation air during 149
the start-up of the combustor. The biomass pellets are fed to the reactor at the location just above the 150
distributor plate by means of a screw feeder. To ensure the fuel feed rate controllable and repeatable, 151
the feeder motor frequency is controlled by an inverter. The flue gas stream leaving the reactor passes 152
through a high efficiency cyclone to recover the elutriated solids and ash and then is exhausted through 153
the ventilation system. The gas composition at the exit of the reactor is continuously analysed by on-154
line gas analysers after going through the water-cooled sampling probe placed inside the gas exit pipe 155
of the cyclone, water condensation traps and particle filters. O2, CO2 and CO concentrations are 156
measured by an Easy line continuous gas analyser (ABB, EL3020), while the NOx concentration is 157
measured by a chemiluminescent NOx analyser (Horiba VA-3000). In order to minimise instrumental 158
errors and drifts, gas analysers are regularly calibrated with BOC/Linde Group calibration gases during 159
each test campaign. The reactor is equipped with pressure tapings and K-type thermocouples at different 160
heights. Both the pressure differential across the dense bed and the temperatures along the reactor are 161
closely monitored during each test so that the signs of agglomeration, defluidisation or extremely high 162
temperature can be spotted at the earliest opportunity. A data taker and computer system are used to 163
7
continuously record all of the measured process data (pressure differentials, temperatures, gas 164
composition, etc.). 165
P-5
T-5
T-3
Cyclone
Screw feeding system
S-2
P-3
P-4
S-3
T-1
T-4
T-6
P-1
S-4
S-1
T-7
P-6
V-4
F-1
Flue gas
Ce
ram
ic h
eat
ers
T-2
Air preheater
P-2
Gas flow controls
Water out
Water in
Heat extraction probe
Data taker
Gas analysers
Computer
Geared motor
Distribution plate
Inverter
Thermocouples (T)
Air staging points (S)
Pressure tapings (P)
F-2
Feeder air
V-3
V-2
V-1
Air
CO2
O2
Gas sampling probe
166
Fig. 1. Schematic representation of 20 kWth BFB combustor experimental setup. 167
168
8
2.2 Biomass fuels 169
Three different types of biomass fuels, two non-woody (miscanthus and wheat straw) and one woody 170
(domestic wood), were tested with the 20kWth BFB combustor under both conventional air combustion 171
and oxy-fuel combustion conditions. All fuels were purchased from the UK suppliers Brites, in the case 172
of the wood, and Agripellets Ltd, in the case of miscanthus and straw, and used in pellet form. 173
Miscanthus pellets have an average diameter of 6.30 mm and an average length of 18.70 mm, with a 174
bulk density of 603 kg/m3. Straw pellets have an average diameter of 6.15 mm and an average length 175
of 15.60 mm, with a bulk density of 628 kg/m3. Wood pellets have an average diameter of 6.00 mm and 176
an average length of 23.10 mm, with a bulk density of 677 kg/m3. The proximate and ultimate analyses 177
of the fuels are shown in Table 1. 178
179
Table 1. Ultimate analysis, proximate analysis and calorific values of biomass fuels*. 180
Miscanthus Straw Wood
Proximate ( wt%, dry basis, except moisture)
Moisture (wt%, as received) 3.34 5.22 3.94
Ash 1.80 6.23 0.70
Volatile matter 82.85 76.31 85.11
Fixed carbon 15.37 17.54 14.19
Ultimate ( wt%, dry basis)
Carbon 45.87 43.80 47.18
Hydrogen 6.74 6.78 6.84
Nitrogen 0.38 0.55 0.17
Sulphur 0.19 0.58 0.17
Oxygen (by difference) 46.82 48.29 45.65
Calorific value HHV, MJ/kg [22] 19.10 18.21 19.81
*Fuels are characterised according to the UK/European solid biofuels’ standards. In particular, volatile matter is 181 determined at 900°C 182 183
2.3 Procedure and operating conditions 184
The operating conditions for all of the runs and the average flue gas compositions at steady state at 185
every condition are summarized in Table 2. Garside 14/25 sand with a Sauter mean diameter (d32) of 186
0.78 mm and a density of 2655 kg/m3 was used as the inert bed material [23]. The sand was added to 187
9
the reactor from the top flange up to a height of 25 cm (3.2 kg). The excess oxygen coefficient levels 188
were maintained roughly constant in all experiments (30%). To investigate the effect of the O2 189
concentration in the oxy-fuel oxidant gas on temperature profiles and gas emissions, a mixture of O2 190
and CO2 gases with various O2/CO2 ratios was produced and used as the main oxidant of each oxy-fuel 191
combustion test. As the overall excess oxygen coefficient/oxygen level must remain constant, variations 192
in the O2 concentration in the oxy-fuel oxidant were hence achieved through variations in the CO2 flow 193
rate and at the same time keeping the O2 flow rate and fuel feeding rate constant in all the experiments 194
(Table 2). The names given to the different oxy-fuel conditions (oxy-21, oxy-25 and oxy-30, as shown 195
in Table 2) are related with the O2 concentration in the oxy-fuel oxidant fed to the reactor under each 196
oxy-fuel combustion test condition (i.e. 21, 25 and 30 vol. %, respectively, as indicated in the 3rd row 197
at Table 2). However, it is worth mentioning that the overall oxygen concentration for the total amount 198
of oxidants fed to the reactor was somewhat different from the indicated O2 concentration in the oxy-199
fuel oxidant as pure air was fed always to the fuel hopper in order to avoid backfiring. The use of air as 200
the feeder gas instead of an oxy-fuel mixture was resulted from safety concerns, i.e. the failure of the 201
interlock between CO2 flow and O2 flow could lead to pure O2 being fed to the fuel hopper. The 4th row 202
of Table 2 indicates the average O2/N2/CO2 concentration of the all gases fed to the reactor for every 203
condition studied. To investigate the effect of the combustion atmosphere, conventional air combustion 204
experiments were also carried out with all the biomass fuels. In addition, at least three runs were 205
performed with each biomass for each condition studied, in order to verify the results achieved. 206
207
Table 2. Operating conditions for oxy-fuel and air combustion experiments and average flue gas compositions at 208
steady state for the different conditions studied. 209
Air Oxy-21 Oxy-25 Oxy-30
Excess oxygen coefficient (%) 30 30 30 30
O2 in Oxy-fuel mixture (vol %) - 21 25 30
Average O2/N2/CO2
concentration of the all gases fed
to the reactor (vol. %)
21/79/0 21/22/57 24/25/52 27/28/45
Feeder air (L/min)1 95 95 95 95
Primary air (L/min)1 250 - - -
10
Total O2 flow (L/min)1 72.5 72.5 72.5 72.5
CO2 flow (L/min)1 - 197.5 157.5 122.5
Total gas flow (L/min)1 345 345 305 270
Superficial gas velocity (U)
(m/s)1
2.56 2.56 2.27 2.01
Fluidization number (U/Umf)2 8.94 8.90 7.90 7.02
Biomass feed rate (kg/h) 3–5 3–5 3–5 3–5
Static height of bed materials
(mm)
250 250 250 250
Average diameter of bed
materials (mm)
0.78 0.78 0.78 0.78
Cooling water flow rate (L/min) 1.2 ± 0.2 1.2 ± 0.2 1.2 ± 0.2 1.2 ± 0.2
Wo
od
O2 (vol. %) 4.72 ± 0.33 NSV3 5.27 ± 0.79 5.49 ± 0.37
CO2 (vol. %) 15.65 ± 0.31 NSV 72.20 ± 2.73 69.14 ± 2.09
NOx (ppm) 69.4 ± 8.3 NSV 86.9 ± 5.9 95.8 ± 3.3
CO (vol. %) 0.47 ± 0.11 NSV 0.52 ± 0.07 0.33 ± 0.02
Mis
can
thu
s
O2 (vol. %) 4.70 ± 0.34 NSV 5.03 ± 0.31 5.63 ± 0.68
CO2 (vol. %) 15.74 ± 0.27 NSV 70.23 ± 2.60 68.44 ± 2.20
NOx (ppm) 111.3 ± 13.6 NSV 156.3 ± 19.2 116.9 ± 7.2
CO (vol. %) 0.57 ± 0.20 NSV 0.31 ± 0.09 0.46 ± 0.13
Str
aw
O2 (vol. %) 5.03 ± 0.30 NSV 4.97 ± 0.21 5.95 ± 0.61
CO2 (vol. %) 15.35 ± 0.65 NSV 70.61 ± 2.05 68.49 ± 0.66
NOx (ppm) 183.7 ± 15.4 NSV 340.1 ± 0.1 243.6 ± 34.2
CO (vol. %) 0.45 ± 0.10 NSV 0.17 ± 0.06 0.16 ± 0.08
1 Calculated at 20°C and 1 atm. 210 2 Both U and Umf calculated at 800°C. 211 3 Not Stable Value. 212
213
Fig. 2 shows the temperature profiles and the gas product distribution at the outlet of the reactor in a 214
typical experiment (in this case firing miscanthus). Each series of tests was always started with 215
conventional biomass air combustion. After stabilisation, which took about 2 hours, the combustion 216
environment was changed from air to oxy-fuel with continuous biomass feeding. After finishing the 217
oxy-fuel tests, the combustion oxidant was switched back to air with continuous biomass feeding 218
operation, minimising any errors resulted from biomass feeding rate fluctuations between the two 219
combustion environments. 220
11
221
Fig. 2. The temperature profiles, and gas concentrations at the outlet of the reactor during miscanthus pellets 222
combustion under air and oxy-fuel conditions. 223
224
3 Results and discussion 225
3.1 Effect of combustion atmosphere on fuel combustion and temperature profiles 226
Fig. 3 shows the temperature profiles along with the height of the reactor under different combustion 227
atmospheres for all fuels. As it can be seen, when using air as oxidant the temperature profile for the 228
non-woody fuels shows a clear maximum located in the splash zone/beginning of the freeboard (T4), 229
above the dense bed, when burned in air. This behaviour happens as a result of the characteristic high 230
volatile matter content of biomass fuels and its release and combustion mostly in the splash zone and 231
freeboard, instead of inside the dense bed as being observed in the case of coal combustion [24]. Other 232
225 250 275 300 325 350 375 400
0
50
100
150
200
NO
x c
on
c. (p
pm
)
Time (min)
T2
T3
T4
T5
T6
T7
0
1
2
3
4
5
CO
co
nc
. (%
vo
l.)
400
500
600
700
800
900
0
20
40
60
80
25 50 75 100 125 150 175
0
50
100
150
200
NOX
CO
O2
CO2
AIROXY-30OXY-25OXY-21
Tem
pera
ture
(°C
)
AIR
CO
2 c
on
c. (%
vo
l.)
0
2
4
6
8
10
12
O2 c
on
c. (%
vo
l.)
NO
x c
on
c. (p
pm
)
Time elapsed after feeding (min)
0
1
2
3
4
CO
co
nc. (%
vo
l.)
12
authors reported similar results [25]. The maximum is not that clear in the case of wood as T2 and T4 233
are quite similar. As the volatile matter content is similar for all the fuels (Table 1) the differences in 234
the temperature profiles between the woody and the non-woody fuels could be partly due to the 235
difference in the content of fines in the fuels. To confirm this theory, the fines content of the three 236
biomass fuels used in this work after passing through the screw-feeder was determined. Fines were 237
assumed to be those particles passing through the sieve with apertures of 3.18 mm. The results obtained 238
showed that wood has significantly less fines content than the non-woody fuels (ca. 4 wt. % for wood 239
and between ca. 8.5-9.5 wt.% for the non-woody biomasses), and this implies that a significantly larger 240
fraction of the woody biomass fuel is expected to be burned in the dense bed of the reactor and hence 241
lead to a higher temperatures in that region in comparison to the non-woody biomass fuels. On the other 242
hand, it can also be seen that usually after the temperature peak, a remarkable temperature decrease in 243
the freeboard is observed in all three biomass fuels, due to the fact that the heat extracted by the water 244
cooling probe from the upper part of the freeboard is much higher than the heat released from the 245
combustion of any unburned fuels within the freeboard. 246
247
13
248
Fig. 3. Temperature profiles for all fuels under air and different oxy-fuel mixtures. The green arrows placed on 249 the left in all graphs indicate continuous drop of temperatures. 250
251
As it can observed from Fig. 2 and Fig. 3, replacing the main combustion air with the oxy-fuel mixture 252
with the same oxygen concentration at 21 vol.% caused a significant drop of all temperatures along the 253
reactor and freeboard, with no steady temperature being reached even after one hour of continuous 254
feeding under the same conditions (the arrows in Fig. 3 indicate continuous drop of temperatures). Due 255
to this constant drop in temperatures the steady biomass combustion could no longer be sustained within 256
the reactor and CO and O2 concentrations increased abruptly, meanwhile NOx concentration showed a 257
sharp decline (Fig. 2). The main reason for the observed temperature decreases is due to the higher 258
specific heat of CO2 compared with that of nitrogen. It is also mentioned in the literature that CO2 plays 259
a significant role in reducing the fuel burning rates by limiting the concentration of O and H radicals 260
during oxy-fuel combustion [26]. In order to continue the experiment, the oxygen concentration in the 261
oxy-fuel oxidant gas was increased before the main combustion temperature decreased to about 500 - 262
550 600 650 700 750 800
0
20
40
60
80
100
120
140
160
180
D
ista
nc
e f
rom
dis
trib
uto
r (c
m)
Temperature (°C)
Air
Oxy-21
Oxy-25
Oxy-30
Wood Miscanthus
Straw
500 550 600 650 700 750 800 850
0
20
40
60
80
100
120
140
160
180
Dis
tan
ce
fro
m d
istr
ibu
tor
(cm
)
Temperature (°C)
500 550 600 650 700 750 800 850
0
20
40
60
80
100
120
140
160
180
Dis
tan
ce
fro
m d
istr
ibu
tor
(cm
)
Temperature (°C)
550 600 650 700 750 800 850
0
20
40
60
80
100
120
140
160
180
Dis
tan
ce
fro
m d
istr
ibu
tor
(cm
)
Temperature (°C)
14
600°C. By increasing the O2 concentration from 21 vol.% to 25 vol.% in the oxy-fuel oxidant gas, a 263
significant increase in the freeboard temperatures (T5-T7) was noticed for all the fuels, as a consequence 264
of both the increase of the O2 concentration (which resulted in a higher oxidation rate of the fuel) and 265
the reduction of the CO2 flow rate, almost matching those temperature values achieved with air 266
combustion and only the temperature values at the beginning of the freeboard (T4) slightly lower than 267
those observed during air combustion (between 30 and 65 °C lower) (Fig. 2 and Fig. 3). However, the 268
main bed temperatures (T2 and T3) remained quite low and were about 100 – 200 °C lower than those 269
observed with air combustion. To almost fully match the temperature profile of oxy-fuel combustion to 270
that of air combustion, the oxygen concentration in the oxy-fuel mixture had to be increased to about 271
30 vol.% as it can be seen from Fig. 2 and Fig. 3. With 30 vol.% O2 in the oxy-fuel oxidant, the 272
temperatures were seen to be slightly higher than those achieved with air, especially for wood and 273
miscanthus. These results agree with literature which suggest the required O2 concentration in the oxy-274
fuel oxidant lies in the range 27 - 30 vol.% [16, 27-29]. 275
276
3.2 Effect of combustion environment on CO emissions 277
The results of CO emissions from different types of biomass fuels tested under air and oxy-fuel 278
combustion are plotted in Fig. 4. The results are expressed in the amount of emission per energy unit 279
(ng/J). This kind of emission unit is commonly used in oxy-fuel literature as the volumetric flow rate 280
of the flue gas of an oxy-fuel combustion plant with real flue gas recirculation is much lower (up to 281
80%) than that of conventional air combustion, and thus the conventional emission unit of volumetric 282
concentration (ppm or vol.%) cannot be used for direct comparison of emissions between two 283
combustion environments. As no steady conditions were reached at oxy-21 conditions, CO and NOx 284
emissions of oxy-21 combustion tests were not going to be compared with those of air combustion and 285
oxy-30 combustion tests both of which had achieved steady-state conditions (Fig. 2). The results of the 286
oxy-25 combustion tests were included for the comparison but it should be noted that these tests were 287
only reached quasi-steady state conditions (Fig. 2). Fig. 4 shows that the switch from air to oxy-25 led 288
a decrease in the CO emission for all the fuels but the reductions in CO emissions were more 289
15
pronounced with the non-woody biomass fuels. As commented in Section 2.3, the increase of the 290
oxygen concentration from 21 vol.% to 25 vol.% in the oxy-fuel atmosphere was achieved through 291
reducing the CO2 flow rate while maintaining the O2 flow rate (in order to maintain constant the excess 292
oxygen coefficient/O2 value), and this resulted in a lower overall fluidizing gas flow rate than that used 293
with air combustion and hence a longer residence time of the combustible gas in the reactor, benefiting 294
the oxidation of CO with O2. However, as it can be seen from Fig. 2 and Fig. 3, the main bed 295
temperatures (T2 and T3) achieved under the oxy-25 combustion conditions were significantly lower 296
than those obtained under the air combustion conditions (between 100 and 200 °C lower as indicated 297
in Section 3.1), due to the aforementioned higher specific heat of CO2 compared to N2. The lower bed 298
temperatures meant more incomplete carbon combustion and as a consequence higher CO emissions in 299
the bed zone, agreeing with the trend observed by Duan et al. [16]. The observation of lower CO 300
emissions with oxy-25 than those of air combustion shown in Fig. 4 further proves that when firing 301
biomass fuels in a FB reactor the main combustion reaction takes place mainly above the dense bed in 302
the splash region and/or at the beginning of the freeboard, while the main bed zone plays a minor role 303
for the combustion of biomass fuels. As shown in Fig. 2 and Fig. 3, for each biomass fuel, the gas 304
temperatures in the freeboard with oxy-25 were slightly higher than those achieved with air combustion 305
and this, together with the longer residence time mentioned above, could have led to the lower CO 306
emissions measured at the exit of the cyclone. The reduction in CO emissions when switching from air 307
to oxy-25 was significant in the case of wood (from 2600 to 1900 ng/J, a 26% decrease) and even more 308
pronounced for the non-woody fuels (from 3250 to 1500 ng/J firing miscanthus, a 54% decrease, and 309
from 2900 to 600 ng/J firing straw, a 80% decrease).The difference observed between the woody 310
biomass and the non-woody fuels could partly be due to the difference in the content of fines in the 311
fuels: as the wood has significantly less fines, an important part of the combustion of this fuel should 312
take place in the low-temperature dense bed zone, and hence resulting in higher CO concentrations in 313
the dense bed zone and an overall less reduction in CO emissions when oxy-25 replaces the air 314
combustion. 315
316
16
As it can also be seen from Fig. 4, increasing the O2 concentration from 25 to 30 vol.% resulted in a 317
further decrease in the CO emissions in the case of wood, while CO emissions slightly increased for 318
miscanthus and slightly decreased for straw. When the O2 concentration in the oxy-fuel oxidant gas 319
reached 30 vol.%, the gas temperature profiles in the whole reactor almost completely matched the 320
temperature profiles achieved with air combustion (as seen in Fig. 2 and Fig. 3), and hence CO 321
emissions were expected to be lower than those of air combustion and oxy-25 combustion as observed 322
with the oxy-combustion of wood. The trends observed with different fuels could be further complicated 323
by the aforementioned differences in the content of fines in the fuels and by the fact that the oxy-25 324
combustion tests had only reached quasi-steady state conditions. Wood has a much lower content of 325
fines comparing with the non-woody fuels, which leads to an increase in the importance of the dense 326
bed on the CO chemistry. 327
328
329
Fig. 4. CO emissions from different types of biomass fuels under air and different oxy-fuel combustion 330 environment. 331
AIR OXY-25 OXY-30
0
500
1000
1500
2000
2500
3000
3500
CO
em
iss
ion
s (
ng
/J)
Wood
Miscanthus
Straw
17
3.3 Effect of combustion environment on NOx emissions 332
The NOx emissions from different types of biomass fuels studied under various combustion 333
environments are presented in Fig. 5. Fig. 5 clearly shows that the NOx emissions’ level depends 334
directly on the nitrogen content of the fuel, indicating that NOx are mainly originated from fuel-N 335
conversions [30]. Prompt-NOx and thermal-NOx weren’t expected to be of importance due to the 336
relatively low combustion temperatures reached at the reactor. It can be seen from Fig. 5 that the NOx 337
emissions increased when the combustion environment was switched from air to oxy-25. In order to 338
explain this behaviour it is necessary to briefly review the fuel-NOx formation mechanism, which can 339
be summarized as follows. As the fuel is fed into the furnace, volatile release takes place. The fuel 340
nitrogen released with the volatiles (volatile-N) further decomposes into NOx precursors such as 341
cyanide (HCN) and ammonia (NH3), which are produced mostly from the amino acid containing fuel-342
N in the pyrolysis process [31]. Furthermore, NH3 can be regarded as the primary product of fuel-N, 343
directly from pyrolysis or the secondary conversion from HCN through reaction R1 [32-35]. 344
CNHHHCN 32 (R1) 345
Depending on temperature and oxygen concentration, NH3 could be partly oxidized to NO (reaction 346
R2) or acts as a reducing agent to reduce NO into N2 (reaction R3). 347
OHNOONH 2232
3
4
5 (R2) 348
OHNNONH 2236
5
3
2 (R3) 349
In this study, biomass was fed at the bottom of the dense bed zone, right above the distributor plate. 350
Therefore, there were various fuel-rich (or oxygen-depleted) pockets existing in the dense bed zone. 351
Within these fuel-rich pockets, a higher NH3 concentration would favour NO reduction (reaction R3). 352
At the same time, a large amount of solid fuel particles existed in the dense bed zone, thus enhancing 353
the NO reduction by CO and char via reaction R4 [36]: 354
222
1CONCONO char (R4) 355
18
As shown in Fig. 2 and Fig. 3, the bed temperatures under the oxy-25 combustion conditions were 356
considerably lower than those achieved under the air combustion conditions, whereas the temperatures 357
of the freeboard region, especially the top part of the freeboard, under the oxy-25 combustion conditions 358
were similar to or slightly higher than those under the air combustion conditions. Therefore, more 359
volatiles were combusted in the oxygen-rich freeboard zone under the oxy-25 combustion conditions 360
than under the air combustion conditions, hence favouring the oxidation of volatile-N such as NH3 and 361
enhancing the formation of NOx via reaction R2. This explains the higher NOx emissions found under 362
the oxy-25 combustion conditions. The results obtained here with the biomass fuels disagree with the 363
common trend found in the literature firing coal. NOx emissions in oxy-coal combustion with low O2 364
concentrations (between 21-25 vol.%) in the oxidant have been found to be usually lower than those 365
observed with air-firing atmospheres, mainly because of the lower temperatures achieved in the bed 366
region at the oxy-coal conditions [11, 37-39]. In the present work, the main bed temperature reached 367
when burning the biomass fuels under the oxy-25 conditions was also lower than that obtained under 368
the air combustion conditions. However, the biomass fuels tested in the present study contain much 369
higher amounts of volatiles (76 – 85 wt. %) than the coals investigated with the previous studies of oxy-370
coal combustion (5 and 36 wt. %) [11, 37-39]. The much higher volatile matter contents with the 371
biomass fuels would mean a larger part of each biomass fuel being burned in the high-temperature 372
freeboard zone in comparison to coal, and accentuates again the decrease in the importance of the dense 373
bed on pollutant emissions when firing biomass fuels. 374
19
375
Fig. 5. NOx emissions from different types of biomass fuels under air and different oxy-fuel combustion 376 environment. 377
378
As shown in Fig. 5, the increase in NOx emissions is more pronounced for fuels with higher fuel-N 379
contents. The increase was seen to be largest for the wheat straw pellet, almost doubling the NOx 380
emissions value obtained under the air combustion conditions (from 160 ng/J to 300 ng/J approx.). On 381
the other hand, when firing wood the NOx emissions remained at almost the same value when switching 382
from air to oxy-25. This is not unexpected as the wood pellet fuel contains a very low level of fuel-N 383
which means other NOx formation routes such as thermal-NOx and prompt NOx can also contribute to 384
the observed NOx emissions. In addition, the wood pellet fuel has a lower fines content and therefore 385
more fuel combustion and NOx formation happen in the dense bed region, weakening the difference in 386
NOx emissions between air combustion and oxy-25 combustion seen with the non-woody biomass 387
fuels. 388
389
AIR OXY-25 OXY-30
0
10
20
30
40
50
60
70
80
90
150
200
250
300
NO
x e
mis
sio
ns
(n
g/J
)
Wood (N content = 0.17%)
Miscanthus (N content = 0.38%)
Straw (N content = 0.55%)
20
Switching from oxy-25 to oxy-30 resulted in a significant decrease of the NOx emissions for the non-390
woody fuels, leading to similar NOx emission levels in comparison to the air-fired tests. As it was seen 391
in Section 3.1, switching from oxy-25 to oxy-30 led to an increase of the dense bed zone temperature, 392
matching that achieved under the air combustion conditions. As a result, more volatile-N was released 393
and converted in the dense bed zone which contains many fuel-rich pockets that favour the reduction 394
of NO to N2 via reaction R3. Furthermore, as explained in Section 2.3, a higher O2 concentration in the 395
oxidant implies a lower CO2 flow rate, in order to keep constant overall excess oxygen 396
coefficient/oxygen in the system. The lower CO2 amount fed to the reactor meant less total oxidant flow 397
to the reactor and therefore less dilution of the gases inside the reactor. Due to this, higher concentrations 398
of NOx reducing species, especially CO, were expected in the dense bed zone which now has similar 399
temperatures to those of air combustion (Fig.2 and Fig.3), promoting the heterogeneous reduction of 400
NOx on the char surfaces, according to reaction R2. Higher O2 concentrations in the bed zone with oxy-401
30 could also promote the oxidation of volatile-N to NOx, hence offsetting the reductions of NOx by 402
CO, char and other NOx reducing species. This may partly explain why the NOx emissions of straw 403
combustion under the oxy-30 condition conditions were found to be slightly higher than those under 404
the air combustion conditions, whereas the opposite trend (i.e. slightly lower) was observed for the 405
miscanthus combustion. A significant portion of the formed CO in the dense bed is burned afterwards 406
in the high-temperature freeboard, leading to lower CO emissions at the outlet of the reactor under the 407
oxy-30 combustion conditions than under the air combustion conditions (Fig. 4). Liu et al. [40] found 408
that the conversion of fuel–N to NOx for coal combustion in 30% O2/70% CO2 was smaller than that 409
for coal combustion in air. They explained this trend in terms of the lower dilution of the gases 410
accomplished when working under the oxy-fuel condition, which increase the concentration of reducing 411
species inside the reactor. Andersson et al. [41] found that the formation of NO from fuel-N in oxy-fuel 412
combustion was the same, or slightly higher, than that under air-firing conditions, whereas the reduction 413
of NO could be up to 50% greater under oxy-combustion conditions. Lupiañez et al. [42] found similar 414
NOx emissions in both air and oxy-fuel conditions (with O2/CO2 ratios of 25/75 and 40/60) in the 415
combustion of lignite in a 90 kW fluidized bed combustor. Díez et al. [43] observed a strong diminution 416
in NOx emissions when the O2 concentration in the oxy-fuel oxidant was increased to 40 and 60%, 417
21
when firing anthracite in a 90 kW fluidized bed combustor. The authors explained this behaviour in 418
terms of the heterogeneous (char, CaCO3, CaO) and homogeneous (CO) interactions of the NOx 419
formed, which can significantly contribute to NOx depletion in the dense bed zone. 420
421
4 Conclusions 422
Three biomass fuels, one woody and two non-woody, have been tested in a 20 kWth fluidized bed 423
combustor. The effect of combustion atmosphere (air or oxy-fuel) and oxygen concentration in the 424
oxidant under the oxy-fuel combustion conditions on temperature profiles and gas emissions (NOx and 425
CO) were systematically investigated. The following conclusions can be drawn from the obtained 426
experimental results: 427
(1) Simply replacing the air with an oxy-fuel oxidant comprised of 21 vol.% O2 and 79 vol.% CO2 428
results in a significant decrease in gas temperatures and ultimately leads to the extinction of the 429
biomass flame due to the larger specific heat of CO2 compared to N2. To keep a similar 430
temperature profile to that achieved under the air combustion conditions, the oxygen 431
concentration in the oxy-fuel oxidant of O2/CO2 mixture has to be increased to ca. 30 vol.%. 432
(2) A drastic decrease in CO emissions can be achieved for all the biomass fuels (up to 80% 433
reduction when firing straw) under oxy-fuel conditions when the oxygen concentration in the 434
oxy-fuel oxidant is 25 vol.% or more as a result of the higher residence time of the gas inside 435
the reactor and the freeboard/reactor temperature profile matching that of air combustion. 436
(3) NOx emissions decrease with the oxygen concentration in the oxy-fuel oxidant due to i) the 437
increase of bed temperature, which implies more volatile-N released and converted in the 438
highly-reducing dense bed zone and ii) the less dilution of the gases inside the dense bed zone, 439
which leads to a higher CO concentration in this region enhancing the reduction of NOx. 440
Similar NOx emissions to those obtained with air combustion were found when the biomass 441
fuels were burned in the oxy-fuel oxidant gas containing 30 vol.% oxygen. 442
(4) The freeboard temperature is the dominant variable influencing both CO and NOx emissions 443
compared with the influence of bed temperature, especially when firing the non-woody fuels. 444
22
Acknowledgment 445
This work was supported by the UK Engineering and Physical Sciences Research Council [grant 446
number EP/M01536X/1]. 447
448
23
References 449
[1] Zhang WB, Liu H, Sun CG, Drage TC, Snape CE. Capturing CO2 from ambient air using a 450
polyethyleneimine-silica adsorbent in fluidized beds. Chemical Engineering Science 451
2014;116:306-16. 452
[2] Lackner KS, Grimes P, Ziock HJ. Carbon dioxide extraction from air: Is it an option? : Coal 453
and Slurry Technology Association, Washington, DC (US); Los Alamos National Lab., NM 454
(US); 1999. 455
[3] Lackner KS, Brennan S, Matter JM, Park AHA, Wright A, van der Zwaan B. The urgency of 456
the development of CO2 capture from ambient air. Proceedings of the National Academy of 457
Sciences of the United States of America 2012;109(33):13156-62. 458
[4] IEA. World Energy Outlook 2016. 2016. 459
[5] Toftegaard MB, Brix J, Jensen PA, Glarborg P, Jensen AD. Oxy-fuel combustion of solid fuels. 460
Progress in Energy and Combustion Science 2010;36(5):581-625. 461
[6] Scheffknecht G, Al-Makhadmeh L, Schnell U, Maier J. Oxy-fuel coal combustion-A review of 462
the current state-of-the-art. International Journal of Greenhouse Gas Control 2011;5:S16-S35. 463
[7] Wall T, Liu Y, Spero C, Elliott L, Khare S, Rathnam R, et al. An overview on oxyfuel coal 464
combustion—State of the art research and technology development. Chemical Engineering 465
Research and Design 2009;87(8):1003-16. 466
[8] Croiset E, Thambimuthu K, Palmer A. Coal combustion in O2/CO2 mixtures compared with 467
air. Canadian Journal of Chemical Engineering 2000;78(2):402-7. 468
[9] Croiset E, Thambimuthu KV. NOx and SO2 emissions from O2/CO2 recycle coal combustion. 469
Fuel 2001;80(14):2117-21. 470
[10] Mathekga HI, Oboirien BO, North BC. A review of oxy-fuel combustion in fluidized bed 471
reactors. International Journal of Energy Research 2016;40(7):878-902. 472
[11] Duan L, Zhao C, Zhou W, Qu C, Chen X. Effects of operation parameters on NO emission in 473
an oxy-fired CFB combustor. Fuel Processing Technology 2011;92(3):379-84. 474
[12] Singh RI, Kumar R. Current status and experimental investigation of oxy-fired fluidized bed. 475
Renewable & Sustainable Energy Reviews 2016;61:398-420. 476
[13] Roy B, Chen LG, Bhattacharya S. Nitrogen Oxides, Sulfur Trioxide, and Mercury Emissions 477
during Oxyfuel Fluidized Bed Combustion of Victorian Brown Coal. Environmental Science 478
& Technology 2014;48(24):14844-50. 479
[14] Hofbauer G, Beisheim T, Dieter H, Scheffknecht G. Experiences from oxy-fuel combustion of 480
bituminous coal in a 150 kWth circulating fluidized bed pilot facility. In: Rokke NA, Svendsen 481
H, editors. 7th Trondheim Conference on CO2 Capture, Transport and Storage. Amsterdam: 482
Elsevier Science Bv; 2014, p. 24-30. 483
24
[15] de las Obras-Loscertales M, Mendiara T, Rufas A, de Diego LF, García-Labiano F, Gayán P, 484
et al. NO and N2O emissions in oxy-fuel combustion of coal in a bubbling fluidized bed 485
combustor. Fuel 2015;150:146-53. 486
[16] Duan LB, Zhao CS, Zhou W, Qu CR, Chen XP. O2/CO2 coal combustion characteristics in a 487
50 kWth circulating fluidized bed. International Journal of Greenhouse Gas Control 488
2011;5(4):770-6. 489
[17] Jia L, Tan Y, Anthony EJ. Emissions of SO2 and NOx during Oxy-Fuel CFB Combustion Tests 490
in a Mini-Circulating Fluidized Bed Combustion Reactor. Energy & Fuels 2010;24:910-5. 491
[18] Saidur R, Abdelaziz EA, Demirbas A, Hossain MS, Mekhilef S. A review on biomass as a fuel 492
for boilers. Renewable & Sustainable Energy Reviews 2011;15(5):2262-89. 493
[19] Jenkins BM, Baxter LL, Miles TR, Miles TR. Combustion properties of biomass. Fuel 494
Processing Technology 1998;54(1-3):17-46. 495
[20] Niu YQ, Tan HZ, Hui SE. Ash-related issues during biomass combustion: Alkali-induced 496
slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, 497
and related countermeasures. Progress in Energy and Combustion Science 2016;52:1-61. 498
[21] Duan LB, Duan YQ, Zhao CS, Anthony EJ. NO emission during co-firing coal and biomass in 499
an oxy-fuel circulating fluidized bed combustor. Fuel 2015;150:8-13. 500
[22] Channiwala S, Parikh P. A unified correlation for estimating HHV of solid, liquid and gaseous 501
fuels. Fuel 2002;81(8):1051-63. 502
[23] http://www.aggregate.com/documents/tds/dried-silica-sand-filler-14-25-tds.pdf. 503
[24] Tarelho LAC, Matos MAA, Pereira F. Axial and radial CO concentration profiles in an 504
atmospheric bubbling FB combustor. Fuel 2005;84(9):1128-35. 505
[25] Kumar R, Singh RI. An investigation in 20 kWth oxygen-enriched bubbling fluidized bed 506
combustor using coal and biomass. Fuel Processing Technology 2016;148:256-68. 507
[26] Molina A, Shaddix CR. Ignition and devolatilization of pulverized bituminous coal particles 508
during oxygen/carbon dioxide coal combustion. Proceedings of the Combustion Institute 509
2007;31(2):1905-12. 510
[27] Pickard SC, Daood SS, Pourkashanian M, Nimmo W. Co-firing coal with biomass in oxygen- 511
and carbon dioxide-enriched atmospheres for CCS applications. Fuel 2014;137:185-92. 512
[28] Liu H, Zailani R, Gibbs BM. Comparisons of pulverized coal combustion in air and in mixtures 513
of O2/CO2. Fuel 2005;84(7):833-40. 514
[29] Tan Y, Croiset E, Douglas MA, Thambimuthu KV. Combustion characteristics of coal in a 515
mixture of oxygen and recycled flue gas. Fuel 2006;85(4):507-12. 516
[30] Winter F. Formation and Reduction of Pollutants in CFBC: From Heavy Metals, Particulates, 517
Alkali, NOx, N2O, SOx, HCl. In: Yue G, Zhang H, Zhao C, Luo Z, editors. Proceedings of the 518
20th International Conference on Fluidized Bed Combustion. Berlin, Heidelberg: Springer 519
Berlin Heidelberg; 2010, p. 43-8. 520
25
[31] Ren QQ, Zhao CS, Chen XP, Duan LB, Li YJ, Ma CY. NOx and N2O precursors (NH3 and 521
HCN) from biomass pyrolysis: Co-pyrolysis of amino acids and cellulose, hemicellulose and 522
lignin. Proceedings of the Combustion Institute 2011;33:1715-22. 523
[32] Liu H, Gibbs BM. Modelling of NO and N2O emissions from biomass-fired circulating 524
fluidized bed combustors. Fuel 2002;81(3):271-80. 525
[33] Shimizu T, Toyono M, Ohsawa H. Emissions of NOx and N2O during co-combustion of dried 526
sewage sludge with coal in a bubbling fluidized bed combustor. Fuel 2007;86(7-8):957-64. 527
[34] Kouprianov VI, Permchart W. Emissions from a conical FBC fired with a biomass fuel. Applied 528
Energy 2003;74(3):383-92. 529
[35] Glarborg P, Jensen AD, Johnsson JE. Fuel nitrogen conversion in solid fuel fired systems. 530
Progress in Energy and Combustion Science 2003;29(2):89-113. 531
[36] Aarna I, Suuberg EM. The role of carbon monoxide in the NO-carbon reaction. Energy & Fuels 532
1999;13(6):1145-53. 533
[37] Jia L, Tan Y, McCalden D, Wu Y, He I, Symonds R, et al. Commissioning of a 0.8 MWth 534
CFBC for oxy-fuel combustion. International Journal of Greenhouse Gas Control 2012;7:240-535
3. 536
[38] Czakiert T, Bis Z, Muskala W, Nowak W. Fuel conversion from oxy-fuel combustion in a 537
circulating fluidized bed. Fuel Processing Technology 2006;87(6):531-8. 538
[39] Duan LB, Sun HC, Zhao CS, Zhou W, Chen XP. Coal combustion characteristics on an oxy-539
fuel circulating fluidized bed combustor with warm flue gas recycle. Fuel 2014;127:47-51. 540
[40] Liu H, Zailani R, Gibbs BM. Comparisons of pulverized coal combustion in air and in mixtures 541
of O2/CO2. Fuel 2005;84(7-8):833-40. 542
[41] Andersson K, Normann F, Johnsson F, Leckner B. NO emission during oxy-fuel combustion 543
of lignite. Industrial & Engineering Chemistry Research 2008;47(6):1835-45. 544
[42] Lupianez C, Guedea I, Bolea I, Diez LI, Romeo LM. Experimental study of SO2 and NOx 545
emissions in fluidized bed oxy-fuel combustion. Fuel Processing Technology 2013;106:587-546
94. 547
[43] Díez LI, Lupiáñez C, Guedea I, Bolea I, Romeo LM. Anthracite oxy-combustion characteristics 548
in a 90 kWth fluidized bed reactor. Fuel Processing Technology 2015;139:196-203. 549
550
551